This invention relates to limited rotation oscillatory motors or galvanometers, and more particularly to that class capable of delivering significant torque (sometimes referred to as torque motors).
A common use of these devices is to drive optical elements such as mirrors for the purpose of guiding light beams. This type of device is often called a scanner. Scanners have a wide angle of rotation, typically more than 15 degrees.
One of the needs in the industry is a strong and accurate high frequency scanner for operation in various electro-optical devices, such as those used in engraving or laser trimming of precision resistors. Higher frequency devices allow for faster and/or more precise operation.
There are a number of types of galvanometers that are known. Common types include the moving coil or so-called D'Arsonval galvanometer, the moving iron galvanometer and the moving magnet galvanometer. Each has its own characteristics and limitations. The invention relates to the moving magnet type.
Moving magnet torque motors fall into three basic types. The first is the imbedded coil design exemplified by U.S. Pat. Nos. 4,302,720 and 3,434,082. They are typically air gap and high inductance devices well suited in ALNICO type magnets. The second type includes a torroid wound on a metal core. Typical of this type are the torque motors marketed by Aeroflex, Clifton and Harowe. Their construction limits them to comparatively low acceleration. Typically they cannot be wound efficiently when their length exceeds a fraction of the diameter. The third type, including the design of U.S. Pat. No. 4,076,988, for example, have a moving magnet device where the coil is stationary and the armature which defines the field rotates. The invention relates to this last class of torque motors.
The parameters that govern the performance of such motors include the torque to inertia ratio, which expresses the acceleration capability, the electrical time constant, which burdens the drive electronics, and the first uncontrollable resonant frequency, which limits the stability of a servo system involving the motor.
Prior designs of moving magnet galvanometers have employed through holes through the magnetic material for clamping the shafts to the magnet. This has been necessitated to assure strong and fixed relationship of the shafts to the magnet material when the magnet is moved as a result of the changing magnetic field imposed upon it.
Certain desirable magnet materials, such as neodymium iron boron, tend to have thermal coefficients of expansion that are different along different axes of the material. Because of this, and because of the varied thermal environments to which these devices are often exposed, a galvanometer rotor magnet made of this type of material may expand significantly in one direction while contracting significantly in the other direction.